1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method, for carrying out a screen processing on an image data. In particular, the present invention relates to an image processing apparatus and an image processing method, for carrying out an averaging processing as a preprocessing of the screen processing.
2. Description of Related Art
On the occasion of outputting image data of a document image, when a screen processing such as a dithering processing is carried out to represent half tone, there are times when a peculiar repetitive pattern called moiré pattern occurs on the outputted image because of the interference between the period due to the halftone dot structure of the document and the period of the pattern applied in the screen processing. Breaking the periodic structure of the document is effective in preventing the occurrence of the moiré pattern, and conventionally, an averaging processing has been carried out for the purpose of breaking such periodic structure (e.g. refer to JP-Tokukaihei-05-324826).
However, the averaging processing averages the periodic structure of the document image as well as the detailed portion of the image, there has been a problem where the sharpness is deteriorated. Therefore, in order to retain the resolution while preventing the presence of the moiré pattern, there has been proposed methods in which the periodicity is broken by dispersing the position which represents the maximum value of the dithering pattern corresponding to the degradation (e.g. refer to JP-Tokukaihei-09-284553), or an edge portion where the sharpness is necessary is detected and an averaging processing is carried out on the edge portion under a condition that is different than other portions (e.g. refer to JP-Tokukai-2000-101845).
However, the conventional averaging processing, which is synchronous to the periodic structure of the screen, was only applicable to the main scanning direction, i.e. only applicable to the one-dimensional case, and was not applicable to the dot-shaped two-dimensional screen. As a result, the moiré pattern is prone to occur for the dot-shaped screen where when a two-dimensional space filter, for example, is used to eliminate the moiré pattern, the sharpness of the output image is lost, causing the occurrence of the deterioration of the image resolution. While a method, which uses a pattern having a periodic structure which is independent to two dimensionalities, is proposed to solve this kind of problems, however, in general, besides that a large memory is needed, a complex calculation of the pixel position must be carried out, causing it difficult to be applied in practice.